An aramide fiber and a polyimide fiber have been conventionally used primarily as a raw material in, for example, aerospace fields and electric fields due to the excellent heat resistance of these fibers. Specific examples of these fibers typically include a para-aramide fiber synthesized from terephthalic acid chloride and p-phenylenediamine, and a meta-aramide fiber synthesized from isophthalic acid chloride and m-phenylenediamine. However, these are produced by a so-called wet spinning method, in which a polymer is dissolved in an acidic solvent or the like, and the dissolved polymer is spun, extracted and then solidified. In this method, special equipments for solvent recovery, pollution prevention, or the like are required, and operations are very complicated.
Also, with regard to a polyimide fiber, a polyimide fiber for use in, for example, aerospace fields or electronic material fields (for example, JP-B-7-42611) and a polyimide non-woven fabric used as a heat resistant bag filter (for example, JP-A-9-52308) has been proposed. However, it is necessary that polyamide acid, an intermediate polymer, is heated to high temperature to be dehydrated and cyclized after fiber spinning and drawing of polyamide acid, there is a problem of not only high production cost but also strength reduction of a fiber due to voids generated after heating. Because of this problem, it is difficult to obtain a practicable nano-order polyimide fiber having high strength and heat resistance.
In batteries and capacitors, a separator is generally laminated between an anode electrode and a cathode electrode. The separator is one of important constituent materials to prevent a short circuit caused by contact of both electrodes. A porous separator is required so as to be provided with ionic conductivity. The separator is used in the form of a non-woven fabric, paper, film, membrane or the like made of such materials as natural fibers, synthetic fibers, glass fibers, synthetic resins or the like, according to each specification of various batteries and capacitors.
A non-woven fabric separator used in electronic components such as batteries and capacitors is produced by a dry production method in which an opened fiber aggregation is uniformly dispersed by a carding machine and the dispersed fibers are combined by thermal bonding or using a binder, or by a wet production method in which fibers as raw material are uniformly dispersed in water to form a paper by a paper machine (refer to, for example, “Latest Technologies of Functional Non-Woven Fabric (CMC Publishing Co., Ltd.), Chapter 14, JP-A-60-52, JP-A-61-232560, JP-A-62-283553 and JP-A-1-258358). However, the non-woven fabric produced by these methods is composed of fibers having a fiber diameter in the range of from several μm to tens of μm, and is therefore unsatisfactory for batteries having high energy density and high electromotive force which are currently desired. Also, because of a large fiber diameter, the obtained non-woven fabric is increased in pore diameter, which enhances the possibility of the occurrence of short circuit between electrodes. If the separator thickness is increased to prevent the occurrence of short circuit, it is difficult to coil the separator and obtain a small-sized and light-weight battery.
With regard to a separator using cellulose fibers of natural fibers as a main component, a non-woven fabric-like sheet is produced by papermaking cellulose fibers and then drying it under heating to remove water. However, it is difficult to improve electric conductivity because voids between fibers are decreased by the influence of surface tension of water when water is removed. In order to solve this problem, dissolving and removing a substance contained in the separator is conducted after the separator is formed, but the process is complicated, resulting in low productivity (for example, JP-A-2000-331663).
A separator of a porous film is produced by a drawing method using polyethylene or polypropylene as a raw material and a method in which an inorganic material is added, and the washing and dissolving using a detergent are conducted. However, the process is complicated and has insufficient safety during reflow soldering or short-circuiting because melting points of polyethylene and polypropylene are about 120° C. and 170° C., respectively.